Portable electronic devices, such as multi-function smart phones, digital media players, and dedicated digital cameras and navigation devices, have display screens that can be used under various lighting environments. Such devices have integrated in them a function that can provide (in real-time) an indication of the current level of visible light in the immediate environment outside the device. This is called an ambient light sensor function (or ALS). The ALS can be used for applications such as automatically managing the brightness of a display screen for better legibility or for conserving battery energy (depending upon the current ambient light level).
Most recently, advanced, consumer grade, ALS integrated circuit (IC) devices have been developed that have a built-in solid state light sensor together with associated analog and digital circuitry that provide, in real-time, a fairly accurate measurement of the ambient visible light that is incident upon the IC device. These IC devices are for the most part manufactured in accordance with a complementary metal oxide semiconductor (CMOS) fabrication process technology.
The response of many typical CMOS light sensor structures (e.g., CMOS photodiodes) is dominated by infrared (IR) content, rather than visible content. This creates a problem under some artificial lighting environments. For example, a CMOS-based ALS device would likely indicate that its ambient environment is “brighter” when lit by an incandescent lamp compared to a fluorescent lamp. That is because incandescent lamp lighting has fairly high IR content compared to, for example, fluorescent lamp lighting, and the sensor cannot distinguish between lighting conditions where IR dominates and those where visible dominates. To help alleviate this problem, an IR blocking filter (IR cut filter) can be placed in front of the sensor, to thereby lessen the sensitivity of the sensor's output to IR content.
In practice of course, the IR cut filter is not ideal in that there still is an appreciable amount of IR content that will pass through the filter and be detected by the sensor. Although relatively small, such IR leakage may still be too much for an ALS in the following situation. Consider the case where a light transparent cover of a portable electronic device is to have a relatively smooth or uniform front surface, without any physical openings therein. An ALS chip is located below the cover, to sense the ambient light level outside the device. In some cases, it is desired to also make the front surface appear dark from outside (e.g., for aesthetic reasons). To achieve this, a layer of IR transmission ink can be applied to the backside of the cover, which gives the front of the cover a uniform, dark color (e.g., black). The IR ink layer, however, allows very little visible content to pass through and reach the sensor below (e.g., no more than about 5% transmission). This diminishes the ability to distinguish IR from visible in the sensor's output signal (despite the reduction in IR content achieved using the IR cut filter).